DESCRIPTION: Mutations in several myofibrillar proteins have been implicated as causes of heritable hypertrophic cardiomyopathies (HCM), and among these, mutations in cardiac myosin binding protein-C (encoded by MYBPC3) are among the most common and have been associated with increased risk for sudden cardiac arrest (SCA) at an early age. However, the cause of SCA is not understood, nor is the observation that some patients expressing mutant cMyBP-C exhibit the hypertrophy and functional sequelae that are characteristic of HCM while others do not. To address these issues, this subproject explores the mechanisms of Ca2+ -triggered arrhythmias in animal models of HCM and also identifies factors such as hypertrophy and co-expression of ion channelopathies that contribute to the profound heterogeneity in the clinical manifestations of disease. The specific hypotheses are: (1) the severity of contractile dysfunction, hypertrophy and SCA stratifies with the degree of cMyBP-C dysfunction, being greatest for C-terminal truncations in which the mutant protein is not incorporated into the myofilaments, (2) the risk of SCA in patients with /WY6PC3rHCM is influenced by the concomitant expression of pro-arrhythmic ion channel polymorphisms, and (3) HCM mutations in MYBPC3, alone or in combination with mutations/polymorphisms in ion channels, cause increased risk of SCA beyond that due to hypertrophy alone. We will test these hypotheses in a series of experiments in which the functional consequences of HCM mutations are studied in living animals and in post-mortem tissue from human HCM hearts. In vivo functional assays will include pressure-volume loops and electrocardiography under resting conditions and during exercise stress testing; in vitro functional assays will include assessment of arrhythmic activity in Langendorff-perfused hearts and both Ca2+ handling and action potentials in isolated cells. The time course and reversibility of effects on contractile and electrophysiological function due to variable expression of HCM mutant cMyBP-C will be studied in transgenic animals in which expression of mutant cMyBP-C is controlled by a tetracycline-inducible system. Results from these studies will provide new insights into the mechanisms underlying disease phenotypes in HCM due to mutations in MYBPC3.